Actually, HVAC P.E. designation didn't exist until after I graduated in 2000 and, quite frankly, HVAC is a watered-downed version of Fluid & Thermal Systems P.E. designation -- maybe they'll get it right some day.
IMHO P.E. peripheral designations exist due to ever developing technology and the increasingly stringent guidlines set forth by various agencies. It only makes sense that people specialize in certain areas, this allows for these people to concentrate on the most efficient means of design with the current technology that meets the required standards. Would you go to a M.E. that primarily deals with HVAC to design a ship for you because it falls under his umbrella? I wouldn't even he did graduate from a top 3 engineering school. What is the sense of being a jack of all trades and master of none?